The present invention relates to a technique that can be effectively applied to an image sensor system using a CMOS image sensor and is intended to reduce power consumption by slowing the speed of image frame processing, for instance a technique that can be effectively applied to camera functions to be mounted on a portable electronic apparatus having a function for communication, such as a mobile telephone.
Image sensors for use in digital cameras, such as video cameras and electronic still cameras, include CCD image sensors and CMOS image sensors. Of these two types, the CMOS image sensor has the advantage of consuming less power than CCD image sensors and therefore suitable for smaller and lighter digital cameras. The CCD image sensor, after transferring in parallel all the accumulated electric charges resulting from the photoelectric conversion of pixels one by one to a transferring CCD at the same timing for all pixels, outputs them within that transferring CCD in series. In order to make the charge transfer efficiency within the CCD high, a generation of a wide difference in potential is required. This invites an increase in power consumption.
On the other hand, the CMOS image sensor converts, on a pixel-by-pixel basis, accumulated electric charges resulting from the photoelectric conversion of pixels one by one into voltages, amplifies the converted voltages, and selects and reads them out consecutively with a matrix selection circuit pixel by pixel. This system can work on a single power source of no more than, for instance, +3.3 V, and its power consumption can be reduced to only a small proportion of that of the CCD image sensor. Furthermore, as CMOS elements can be manufactured by a CMOS process, peripheral circuits including amplifiers and A/D converters can be integrated together, and therefore the number of parts can be reduced.
Because of these advantages, CMOS image sensor-based camera systems have come to find use in recent years where the requirement for small size and light weight is especially keen, such as mobile telephones and mobile data terminals. CMOS image sensors have also come to be used in video camera systems of various types including that of the National Television System Committee (NTSC). To meet these needs, CMOS image sensors with an electronic shutter release function in which peripheral circuits including amplifiers and A/D converters are integrated and the duration of accumulation of electric charges of pixels can be set from outside on a frame-by-frame basis have become available in the market.